The ordinary one-gallon paint can has long been made of steel.
In spite of persistent shortcomings of steel, such as corrosion, particularly when used for aqueous-based paints, a plastic container has not been developed which has proven capable of displacing the standard steel can from the marketplace. The difficulty is not one of can design but of mold design.
Letica, for example, in U.S. Pat. Nos. 4,349,119 and 4,293,080, illustrates reinforced plastic can designs which are different from the standard steel paint can design and therefore would require costly changes in the filling and packaging lines of existing paint factories. The mold must have a collapsible core, which inevitably will manifest mechanical difficulties.
While Holt in U.S. Pat. No. 3,977,563 does not require a collapsible core since he does not employ an inwardly-protruding lip for the lid, this significant departure from existing paint can design would require major changes in the filling and packaging lines of the paint companies, since the lids are not essentially the same as those for the standard metal can.
Von Holdt, in U.S. Pat. Nos. 4,512,494 and 4,286,766, illustrates a collapsible core which may be removed in spite of the inwardly projecting lip which he recognizes as necessary. However, the collapsible core is complicated and susceptible to mechanical breakdown and maintenance problems; moreover, Von Holdt finds it necessary to taper the entire mold.